1. Field of the Disclosure
This invention pertains to a method for thermally developing a photosensitive element, and particularly to a method for forming a relief printing form from the photosensitive element.
2. Description of Related Art
Flexographic printing plates are well known for use in printing surfaces which range from soft and easy to deform to relatively hard, such as packaging materials, e.g., cardboard, plastic films, aluminum foils, etc. Flexographic printing plates can be prepared from photosensitive elements containing photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. The photopolymerizable compositions generally comprise an elastomeric binder, at least one monomer and a photoinitiator. Photosensitive elements generally have a photopolymerizable layer interposed between a support and a coversheet or multilayer cover element. Upon imagewise exposure to actinic radiation, photopolymerization of the photopolymerizable layer occurs in the exposed areas, thereby curing and rendering insoluble the exposed areas of the layer. Conventionally, the element is treated with a suitable solution, e.g., solvent or aqueous-based washout, to remove the unexposed areas of the photopolymerizable layer leaving a printing relief which can be used for flexographic printing. However, developing systems that treat the element with a solution are time consuming since drying for an extended period (0.5 to 24 hours) is necessary to remove absorbed developer solution.
As an alternative to solution development, a “dry” thermal development process may be used which removes the unexposed areas without the subsequent time-consuming drying step. In a thermal development process, the photosensitive layer, which has been imagewise exposed to actinic radiation, is contacted with an absorbent material at a temperature sufficient to cause the composition in the unexposed portions of the photosensitive layer to soften or melt and flow into an absorbent material. See U.S. Pat. Nos. 3,060,023 (Burg et al.); 3,264,103 (Cohen et al.); 5,015,556 (Martens); 5,175,072 (Martens); 5,215,859 (Martens); and 5,279,697 (Peterson et al.). The exposed portions of the photosensitive layer remain hard, that is do not soften or melt, at the softening temperature for the unexposed portions. The absorbent material collects the softened un-irradiated material and then is separated from the photosensitive layer. The cycle of heating and contacting the photosensitive layer may need to be repeated several times in order to sufficiently remove the flowable composition from the un-irradiated areas and form a relief structure suitable for printing. After such processing, there remains a raised relief structure of irradiated, hardened composition that represents the irradiated image.
Processors for thermal development of flexographic printing elements are known. U.S. Pat. No. 5,279,697 and U.S. Pat. No. 6,797,454 each describe an automated process and apparatus for handling an irradiated printing element and accomplishing repeated heating and pressing to remove the unirradiated composition from the element with a web of absorbent material. Both apparatuses include a drum for supporting the photosensitive element during thermal development. An outer surface of the drum may be tacky to temporarily adhere the photosensitive element to the drum. The photosensitive element is mounted on the drum with the element residing on the tacky layer and, during thermal development the absorbent material contacts an exterior surface of the photosensitive element. The preheating drum can be coated with a silicone rubber composition layer having a Shore A hardness rating between about 30 and 60. The rubber coating may be impregnated with aluminum particles. The resilient surface provide by the rubber can result in a longer nip zone as a result of the surface deflecting under the pressure exerted by a hot roller. The hot roll supports the absorbent material in contact with the photosensitive element to heat the element and remove the molten polymer with the absorbent material.
U.S. Patent Application Publication 2005/0142494 A1 describes a method and apparatus for thermal development in which a photosensitive element is supported on a base member with a removable flexible support member. In one embodiment, the flexible support member need not include a carrier and can be formed of a layer of a self-supporting material, such as a compressible material. U.S. Patent Application Publication US 2006/0029880 A1 describes a method and apparatus for thermal development in which a photosensitive element is supported on a base member. U.S. Patent Application Publication US 2006/0134557 A1 describes a method and apparatus for thermal development in which a photosensitive element is supported on a base member having an exterior textured surface. These publications describe that the base member (or the support member) can include a resilient layer. The resilient layer can be composed of any material suitable to provide a Shore A hardness between about 30 and about 75, such as natural rubbers and elastomeric materials and synthetic rubbers and elastomeric materials, including rubber, silicone rubber, and compressible foams.
International Publication Number WO 2007/012023 A2 describes a method and apparatus for processing flexographic printing plates. In one embodiment, the apparatus may include a conformal thermally conductive cushioned surface located between a flexographic plate and a support plate to create a cushioned surface to support the flexographic plate.
In two embodiments of a thermal processor, commercially identified as CYREL® FAST 1000TD and CYREL® FAST TD4260 processors, an exterior surface of the drum includes a silicone rubber layer having a Shore A hardness of 50, and a tackification layer on the rubber layer. The tackification layer is DOW 236, a silicone dispersion in solvent, available from Dow Corning.
A problem sometimes arises with thermal processing in that the uncured photopolymer is not always adequately cleaned out or removed from recessed areas of the relief surface of the printing element. Inadequate clean out of recessed areas can manifest as insufficient removal of photopolymer material to the relief depth desired and/or as a non-uniformity of relief depth between open floor areas and in relatively narrow channels or gaps between large raised areas (i.e., typically solid printing areas). Relief depth is the difference between thickness of a floor of cured polymer and the thickness of the printing layer in the printing element.
It is particularly difficult to adequately clean out or remove uncured photopolymer material in relief printing forms in which the photopolymerizable layer has a thickness greater than about 100 mil, and becomes progressively more difficult as the thickness of the photopolymerizable layer increases. The thickness of some photopolymerizable relief printing forms can be as much as 250 mils or more. Thick relief printing forms are used primarily in printing of corrugated board, sometimes referred to as corrugated paperboard. The relief depth for the thick printing forms can be as much as about half of the thickness of the photopolymer layer.
Oftentimes, in order to achieve improved relief uniformity the pressure at the nip between the hot roll carrying the absorbent material and drum carrying the photosensitive element can be increased to impress the absorbent material into the recessed areas. Increased nip pressure compresses the element creating a wider contacting zone for the nip, i.e., footprint, on the element along an axial length of the drum as well as increases the residence (i.e., dwell) time for the transfer of heat to the element. However, increased residence time that raises temperatures at or above the glass transition temperature of the base support for the element, can result in distortion or deformation of the resulting printing form or plate. Deformations include waves of localized distortions resulting in a non-planar topography of the photosensitive element. Waves of distortions can form in different locations in each element processed. In addition the hot roll has a tendency to deflect or bow along its length, resulting in non-uniform application of pressure to the photosensitive element along axial length of the contact zone. Non-uniform application of pressure can contribute to the resulting relief structure of the printing form having non-uniform relief depth.
Relief printing forms having incomplete clean out or removal of recessed areas of the relief surface can result in poor print performance, that is, poor reproduction of the image printed on the substrate. Relief printing is a method of printing in which the printing form prints from an image area, where the image area of the printing form is raised and the non-image area is depressed or recessed. Recessed areas, such as the floor, that are not cleaned out sufficiently are shallow, and thus can pickup ink and contact to transfer the ink onto the substrate in regions that are not to be printed. This is sometimes referred to as “printing the floor”. In other instances, small dirt particles or lint can cling to the shallow recessed areas, pickup ink, and transfer ink (with or without the dirt particle) to the substrate, which can render the printed image “dirty”. This effect of printing shallow floors or dirt can be exacerbated since the printing form is often in pressure contact with the substrate.
Relief printing forms having distortion/s in the support and/or the cured photopolymeric layer also result in poor print performance. In multicolor printing, when one or more of the relief printing forms have distortion the printed image has poor registration. Even in single color printing, distortion in the relief printing form may print an image that is not an accurate reproduction of its original, so called image infidelity, by printing straight lines as curves for example. The relief printing form having distortion/s may also incompletely print the image due to intermittent contact of the inked surface of the printing form to the printed substrate.